1. Field of the Invention
The present invention relates to a high-speed tool steel gear cutting tool (hereunder referred to simply as a gear cutting tool) in which fractures or chipping (minute fractures) do not occur at the cutting edge even when performing gear cutting at high-speed, and which realizes excellent cutting performance over long periods, and a manufacturing method therefor.
2. Description of the Related Art
Heretofore, in the gear cutting of tooth profiles for various gears used as constituent members for automobiles, aircraft, and various drive units, a gear cutting tool such as for example the hob (solid hob) shown in FIG. 1, a pinion cutter, or a shaving cutter are used.
Furthermore, manufacture of this gear cutting tool by the following steps as illustrated hereunder in (a) to (e) it also well known.
(a) Hot forge an ingot of high-speed tool steel at a temperature of 1100 to 1150° C. to make a bar with an outer diameter of 50 to 150 mm.
(b) Fully anneal the bar stock and then cut to a predetermined length, mill, and rough treatment into a tool material of a shape corresponding to the shape of the final gear cutting tool.
(c) Heat and hold the tool material at a temperature of 1210 to 1270° C. in an atmosphere of nitrogen, and then quench the tool material by blow cooling with pressurized nitrogen gas, to transform the structure of the tool material into martensite.
(d) Heat and hold the post quenched tool material at a temperature of 500 to 550° C. in an atmosphere of nitrogen to temper the tool material and transform the residual austenite dispersingly distributed in the matrix of the martensite structure formed in the quenching into martensite.
(e) Finish the post quenched and tempered tool material into a final shape by for example grinding or polishing.
Furthermore, as the abovementioned gear cutting tool, there is also known a coated gear cutting tool as disclosed for example in Japanese Patent Application, First Publication No. Hei 7-310173, where the surface of the high-speed tool steel base metal is physical vapor deposited to an average thickness of 2 to 15 μm with a hard coating layer comprising either or both of; a composite nitride [hereunder denoted by (Ti, Al) N] layer of Ti and Al and a composite carbonitride [hereunder denoted by (Ti, Al) CN] layer of Ti and Al, for which in the case where these are expressed by composition formula: [Ti1−xAlx]N and composition formula: [Ti1−xAlx]C1−mNm, the atomic ratio measured using an Auger spectroscopy analysis apparatus, of a central portion in the thickness direction satisfies x: 0.30 to 0.70, m: 0.6 to 0.99.
This coated gear cutting tool is shown for example in outline in FIG. 2, and is manufactured using a cathodic arc ion plating apparatus being one type of physical vapor deposition apparatus. In this case, for example the interior of the apparatus is made a vacuum atmosphere of 20 m torr, and in a condition heated with a heater to a temperature of 500° C., an arc discharge is generated under conditions of for example voltage: 35 V, current: 90 A between an anode electrode and the cathode electrode on which is set a Ti˜Al alloy having a predetermined composition. At the same time, a nitrogen gas, or a nitrogen and methane gas, is introduced to inside the apparatus as the reaction gas, and a bias voltage of for example 200 V is applied to the base metal comprising the high-speed tool steel (hereunder simply base metal) so that the hard coating layer is physical vapor deposited on the surface of the base metal.
However, in recent times FA (factory automation) of gear cutting apparatus has become remarkable and there is a strong demand for labor saving and energy saving in the gear cutting process and a reduction in cost. Together with this, there is a demand for generality to enable a variety of gear cutting processes to be performed with only one type of gear cutting tool, and there is also a trend towards speeding up the gear cutting process.
As a result, in the conventional gear cutting tool, in the case where this is used in gear cutting under normal conditions there are no problems. However when used in high-speed gear cutting, this is susceptible chipping, particularly at the ridge line intersection of the rake face and the relief face of the cutting edge, so that the useful life is reached in a comparatively short time.
On the other hand, in the case of the conventional coated gear cutting tool, in the case where this is used in gear cutting under normal conditions, with carbon steel or cast iron or the like there are no problems. However when used in high-speed gear cutting of gears such as low-alloy steel or mild steel with extremely high viscosity, since the affinity between the chips produced by the cutting and the (Ti, Al) N layer or the (Ti, Al) CN layer constituting the hard coating layer is high, the chips are susceptible to adhering to the surface of the cutting edge of the gear cutting tool. This adhering phenomena becomes remarkably apparent the higher the speed of the gear cutting process, and with this adhering phenomena as the cause, fracture and chipping occurs at the cutting edge, so that the useful life is reached in a comparatively short time.